The present invention relates generally to systems which provide variable time division communication between stations connected to a common communication channel and, more specifically, to such systems which provide for immediate acknowledgement of global messages (i.e. messages for communication to all stations connected to the communication channel), to such systems wherein each station on the bus can initialize the system after the system has been quiet beyond a predetermined amount of time, and to such systems which are capable of transmitting multiple messages.
Information communication systems have been used in a wide variety of applications. For example, one important application is in the control and supervision of building functions such as energy management, fire and security, and access control. Any one or all of these building control and supervision functions can be performed by the system installed within the building.
In the energy management area, the system is designed to monitor various status points such as temperature sensors, flow rate sensors, and humidity sensors and to control various command points such as dampers, fans, and the control points of local loop controllers.
Such systems typically comprise a plurality of stations connected to a common communication channel and supervised by a processor. Multiple processors may be connected to another common communication channel so that they can share supervisory and control duties. Such processors can themselves be considered stations but of a higher level than the stations which interface directly with the points being supervised and controlled. If the system is large enough, the processors themselves can be connected to other processors to help share the control and supervisory duties.
If these multiple processing stations are to share control and supervisory duties, it is apparent that they must communicate with one another. In order to provide for an orderly communication system so that more than one station does not try to communicate on the bus at the same time resulting in garbled messages, a communication protocol must be provided. This communication protocol can be as simple as a polling system wherein a central processor establishes direct communication with the various stations connected to the communication channel by polling and commanding the station individually, or the protocol may be more complicated as in the case of a time division multiplexing system in which all of the stations have a specific time slot within which they can transmit messages over the communication channel with no time slot being assigned to more than one station.
Time division multiplexing, where the time division is of fixed or variable duration, is not a new concept. Moreover, prior art systems using time division multiplexing have had provision for the transmission of global messages to stations on the common communication channel. A global message is a message which all stations on the communication channel might find useful or necessary. Even though all stations on the communication channel can find the global messages useful, such global messages usually require a specified station to acknowledge the correct receipt of the global message. The specified station in these prior systems provided such acknowledgement only in its own time slot. However, one of the problems with this kind of system is that the station which transmitted the global message had to wait for the acknowledger's time slot to find out whether it correctly transmitted the global message.
In a time division multiplexing system, and initializing sequence must be provided in the event that the communication channel has been quiet beyond a predetermined amount of time. For example, if a station fails to transmit during its time slot and the other stations on the communication channel resynchronize themselves to the station which is presently transmitting, the communication channel may become permenently quiet unless an initializing sequence is performed. In prior art systems, the initialization of a quiet bus was typically performed by a designated station. In such a system, if the initializer station is the station which failed, the bus can become permanently quiet.
Additionally, prior art time division multiplexing systems were limited to the transmission of one message during the time slot.